Pollution of water with fecal material, whether infected or not, is obviously undesirable, both from the standpoint of its possible danger as a source of infection and for purely esthetic reasons. The detection in water of fecal bacteria of any kind is therefore of importance in determining its suitability for drinking purposes. One very common, readily cultivated and numerically predominant type of intestinal organism is the Escherichia coli (E. coli). In the United States E. coli is considered a sensitive indicator of pollution.
In order to determine its presence, advantage is taken of the ability of E. coli to ferment lactose with the production of gas. But, the Aerobacter and also E. freundii ferment lactose with gas production and it is therefore of importance to distinguish between the three because E. coli is definitely an intestinal organism while Aerobacter and E. freundii are also frequently derived from soil.
The American Public Health Association has outlined an official procedure for the isolation of Escherichia and Aerobacter from water and has described an unofficial but useful procedure for the differentiation between E. coli of truly intestinal nature and closely related, but less frequently intestinal, organisms like E. freundii and the Aerobacter. Briefly, the procedure is as follows:
The water to be examined is inoculated in measured amounts into tubes of extract 0.5% lactose broth, and is incubated for 24 hours. If gas is formed it is regarded as presumptive evidence that E. coli is present.
However, the gas may be due to Aerobacter or other organisms such as Clostridium perfringens, yeasts or synergistic combinations capable of producing gas from lactose. The broth culture is streaked in an agar medium containing some dye which inhibits all but the Escherichia and Aerobacter. If, after inoculation of these liquid cultures, gas is produced, it is strongly suggestive of the presence of Aerobacter or or Escherichia because most other species are inhibited by the dyes. The liquid dye cultures may, for further proof, be streaked on the dye-containing plates as mentioned above. Finally, addition tests are made, to distinguish the Aerobacter from the Escherichia, and E. coli from E. freundii. See Frobisher: Fundamentals of Bacteriology, pp. 431-453, W. B. Saunders Co. (Philadelphia 1944).
The shortcomings of this method are the long time of fermentation needed to get results (particularly for samples which contain small amounts of bacteria, the usual case), the need for visual observation and complicated laboratory procedure. In a typical testing laboratory, hundreds of thousands of samples are run daily.
In the prior art, there have been various approaches to decrease monitoring time and to make monitoring automatic. One prior art method teaches use of impedance changes within the medium to measure growth of various bacteria. Any changes in the electrical resistance between two electrodes in the culture medium are related to the amount of bacteria present. More information on this subject may be found in Impedance Changes in Media as a Method of Identifying Microorganisms, Lawless, Dufour and Cady; Bactomatic, Inc.; Palo Alto, Cal.; 1973.
Another approach is to use isotope labelling of the nutrient medium. For example, if a sugar containing some carbon-14 is used as a nutrient, bacteria present will metabolize the sugar and produce labeled carbon dioxide. The instrument determines the presence and measures the amount of the isotope in the atmosphere above the culture. This technique is more fully described in U.S. Pat. No. 3,676,679.
A third approach is to continuously circulate the culture media through a glass tube and measure changes in turbidity as bacteria multiply.
However, each of the above methods has the disadvantage of either taking a good deal of time or requiring expensive equipment or both.